4x5 Vlads Test Target
How to Use 4x5” Targets:
Place the 4x5 sheet with targets into a film holder before starting the digitizing session. The target has been designed to help with image stitching if needed. If you plan to capture your LF frames in 4 or 6 shots and stitch them together, the numbering on the target should help you rehearse the process and establish a stitching routine.
Capture full target images or fragments and review them on a computer screen to ensure:
Critical focus is achieved. This means that the center of the image, edges, and corners are equally sharp, resolving the same Group and Element as in all USAF 1951 patterns across the frame. The Siemens stars should be equally sharp, and the individual strokes in the targets should be well-defined with clearly defined edges. The fine raster, which is very uniform across the frame, helps ensure uniform sharpness.
Full alignment between the film holder and the camera is attained. This ensures that all elements of the target come into focus simultaneously, with no geometric issues, and all right angles remain accurate.
The optimal F-stop is dialed in for maximum sharpness across the frame. Typically, the best sharpness can be attained at a very specific F-stop, such as 5.6 ½ for the APO Rodagon. Deviation from this optimal value can lead to a deterioration in image quality.
Correct framing is established. Depending on the desired coverage area, the framing should be set to cover the frame.
The movement of the camera or film holder for the purpose of later stitching does not affect sharpness. You should not correct focus if you plan to stitch images. The fine raster will help you ensure that the lighting is uniform across the whole rig. If the size of raster elements changes from shot to shot, this typically signals that lighting is not uniform and requires your attention.
These targets are designed to assist with adjustments if any of the above points become problematic.
The absolute resolution of Type 120 and 4x5" targets is 43 line pairs per mm.
